Gypsum based boards produced from gypsum, such as gypsum boards, plaster plates and so forth, are commercially and widely available as architectural interior finish materials and so forth. The gypsum may be generally classified into gypsum dihydrate, gypsum hemihydrate and anhydrous gypsum, in accordance with existing state of combined water. In general, the gypsum hemihydrate obtained by calcination of the gypsum dihydrate is used as a raw material for production of gypsum based boards. An oven-type furnace (direct-heating-type furnace), an indirect-heating-type furnace and so forth are used as gypsum calciners for producing the gypsum hemihydrate and so forth, as described in Patent Literatures 1 and 2 (European Patent Laid-Open Publication No. 0230793 and Japanese Patent Publication No. 2571374). In general, in a case of the calciner for producing the gypsum hemihydrate from the gypsum dihydrate, an internal temperature of the calciner is set to be a value approximately in a temperature range from 100 degrees C. to 250 degrees C. Further, the calciner for calcining the gypsum hemihydrate to the anhydrous gypsum is known in the art as described in Patent Literature 3 (Japanese Patent Laid-Open Publication No. 2005-15263). In general, in a case of calcination for calcining the gypsum hemihydrate to the anhydrous gypsum, the internal temperature of the calciner is set to be a value approximately in a temperature range from 300 degrees C. to 950 degrees C.
In regard to such kinds of calciners, a conical-kettle-type calciner and a cylindrical calciner are known in the art. The conical-kettle-type calciner has an inverted conical or cone shaped bottom part, whereas the cylindrical calciner has an substantially uniform circular cross-section over its whole height. In recent years, the conical-kettle-type calciner having a bottom part with a reduced diameter tends to be employed for production of the gypsum hemihydrate and so forth. In both type calciners, the calciner is provided with a tubular combustor or an internal tube depending from a top part of the calciner to a center area inside of the calciner, and a raw gypsum supply means, an exhaust gas tube and so forth are connected to the top wall of the calciner.
Such a tubular combustor of the calciner is connected with a fuel supply tube and a combustion air supply duct. A combustion gas or hot gas at a high temperature (referred to as “high temperature gas” hereinafter) is produced by a combustion reaction of a fuel and combustion air, and the high temperature gas is ejected from a lower part of the tubular combustor to a bottom part of the calciner. The solid matters including gypsum dihydrate and so forth, which are accumulated in the calciner, are fluidized by the high temperature gas spouting flow ejected at a high velocity toward the bottom part, and lose the combined water by heat transfer contact with the high temperature gas, whereby the raw material is calcined to gypsum hemihydrate (or anhydrous gypsum). The calcined gypsum thus obtained is extracted from the calciner through an outlet port for calcined gypsum, which is positioned at a certain position of an interior wall surface of the calciner.